iPhone encryption stops FBI, but not this 7-year-old

Matthew Green's iPhone was safely locked -- until his 7-year-old son Harrison figured out how to grab his dad's finger to unlock it.

When it comes to privacy, Matthew Green is a guru. This college professor knows all about NSA spying, encryption, computer security -- the works.

Yet he's met his match: his 7-year-old son.

You see, Green owns an iPhone 6 Plus. That means everything on the phone is encrypted until Green -- and only he -- unlocks it with his finger.

But Tuesday morning at dawn, little Harrison crept into his parents' bedroom and walked over to his dad's side of the bed. He quietly reached for his father's iPhone, grabbed his right hand and pressed his large thumb onto the fingerprint scanner.

Green had this to say on Twitter that morning: "Dear FBI: next time you say 'think of the children' I implore you to note how easily mine can bypass Apple encryption using physical attacks."

Green explained to CNNMoney why biometric features -- like fingerprints or voice -- aren't effective if you want to keep someone out of your phone. A police officer could just press your finger down onto the scanner.

"This is a really serious problem," Green said. "In a situation where you're under arrest, biometrics are not very good [at protecting your information.]"